From statement I: D is the son of C, who has two sons, but no daughter.
C is either mother or father of D.
We cannot determine the relation between A and D.
Hence, the data in statement I alone are not sufficient to answer the question.
From statement II: A's mother is married to C.
We can conclude that C is A's father
We cannot determine the relation between A and D.
Hence, the data in statement II alone are not sufficient to answer the question.
By combining I and II
We can conclude that A is the brother of D.
Hence, the data in both statements I and II together are sufficient to answer the question.